The present invention relates to the field of microwave circuits and fiber optics. More specifically, it relates to injecting a microwave local oscillator (LO) signal into a dual-gate MESFET oscillator circuit from a remote source to create a dual-gate, injection-locked, self-oscillating mixer (SOM).
Currently, FETs are widely used in constructing receiver front-ends. In the past, FET receiver front-ends generally required three FET's to perform three functions (or stages): (1) a preamplifier, (2) a local oscillator and (3) a mixer. It has been shown, however, in an article entitled "12 Ghz Receiver with Self-Oscillating Dual-Gate Mesfet Mixer" by Tsironis, C , published in Electronics Letters, Vol. 17, No. 17, 20th August 1981, that one dual-gate GaAs FET can be configured to provide both the local oscillator and the mixer functions of such devices. A receiver utilizing a dual-gate FET could economize the overall receiver architecture and thus improve its overall performance. In particular, such architecture would reduce the overall conversion loss (IF power/RF power) and noise figure (S/N out/S/N in) of the receiver.
There are, however, some big disadvantages to utilizing such dual-gate self-oscillating mixers in receiver front-ends. For one, receiver performance can be greatly diminished when operating under adverse environmental conditions such as extreme temperature or vibration. This is mainly due to the fact that the receiver LO is local to the receiver, and thus directly subjected to the adverse conditions. Such adverse conditions cause the LO source to become unstable which, in turn, causes the mixer and ultimately the receiver to become unstable.
Another disadvantage, is the inability to synchronize an array of receivers utilizing such dual-gate FET's to the same LO frequency. Since each receiver front end has its own LO, it is impossible to synchronize and tune each to the same frequency.
It has been shown, however, in an article entitled "GaAs FET Applications for Injection-Locked Oscillators and Self-Oscillating Mixers" by Tajima, Y, published in IEEE Transactions on Microwave Theory and Techniques, Vol. 27 no. 7, pp. 629-632, July 1979, that injection-locking can solve these problems associated with dual-gate FETs having a local LO.
Essentially, injection-locking is forcing an FET to oscillate at a desired frequency. In the above disclosure, injection-locking is achieved by feeding electrical energy at a predetermined LO frequency into a single-gate FET that is electrically tuned through a feedback element between its drain and gate port to oscillate at a predetermined frequency. Feeding the electrical energy at the predetermined LO essentially locks the single-gate FET to that predetermined frequency. As such, the single gate FET provides a stable LO frequency source to the mixer stage.
There are many other benefits and advantages to those who utilize injection locking in receiver front-ends. One advantage is that injection-locked FETs can be synchronized to a remote LO source. This provides the ability for an array of receivers to be synchronized to the same frequency. In addition, this eliminates the threat of any adverse environmental effects on the LO and/or the receiver.
A big disadvantage, however, is that receivers utilizing the single-gate injection-locked LO source disclosed above would require an additional FET stage to perform the mixer function. In other words, such receivers would not enjoy the benefits of those utilizing a dual-gate SOM (also described above). Particularly, receivers utilizing the single-gate injection-locked LO source would not have the low conversion-loss and low noise figure enjoyed by those receivers utilizing the dual-gate SOMs.
Consequently, the need for a dual-gate SOM having remote injection-locking capability has long been recognized by those in the field. Receivers utilizing such a device would have the low conversion loss and low noise figure as well as a stable LO under adverse environmental conditions. Moreover, receivers utilizing such a device could be synchronized to the same LO. The present invention fulfills this need.